This invention relates to the production, by a continuous production process, of molten glass exhibiting high-level fining and homogeneity qualities, such as the level meeting the requirements for making flat glass and more particularly float glass.
It relates to a process and an improved device for fining and homogenizing of molten glass, able to be applied after the melting of the vitrifiable mixture both by flames and electrically, but more especially suited to this latter case, because an electrically melted mixture benefits more from its advantages.
It also relates to a complete installation for producing fined and homogenized glass, in which the energy is essentially supplied electrically, thus meeting a very real need, resulting from the advantage, on the cost level, that electrical energy may be of nuclear origin over other energy sources, in particular, hydrocarbons.
Finally, it relates to the applications of this device, particularly for producing glasses from charges with very volatile ingredients whose losses it makes it possible to minimize and, consequently, to reduce both the cost of raw materials and pollution of the atmosphere.
Numerous devices for manufacture of molten glass by use of electric energy have already been proposed so far.
Some essentially proceed from an adaptation of flame furnaces and, for example, in a long tank, such as that of present industrial flame furnaces for flat glass, they provide a certain number of electrodes placed, as might be expected, to reproduce in said tank approximately the same convection currents that are found in said industrial flame furnaces, to make it possible to obtain the continuous progress of the various phases of the process for producing glass suitable for being delivered to a shaping device, namely, melting itself, fining, homogenizing and cooling down. These electric devices require a relatively complex and costly infrastructure, their energy efficiency is generally only on the same order as that of standard flame furnaces, and the shift of know-how in case of changing the scale of production capacity for a given type of installation, is often tricky.
The device described by the patent FR No.-A-2 261 233, for example, which comprises a long tank fed with vitrifiable materials at the upstream end and delivering molten glass at the downstream end suitable for use in a flat glass installation, is heated by a series of rows of vertical electrodes going through the bottom and placed crosswise to the length of the tank. The major convection currents used in this tank originate, as in flame furnaces of this type, in the hottest zone of the glass, or source point, localized by means of at least one, and preferably two, crosswise rows of vertical electrodes, which extend over practically the entire width of the tank to cause the ascending movement of a column of hotter glass, which spreads in two flows, on reaching the surface of the bath, to form two major ringlike thermal currents. The first flow, directed upstream, will lick the lower surface of the composition batch, be cooled, plunge toward the bottom and move downstream, in the lower part of the bath, to the source point, where it is picked up by the ascending current, thus forming a ringlike current, designated hereafter by the expression "melting belt."
The second major surface flow, directed downstream in the furnace, goes beyond the fining zone, the homogenizing and cooling zones, generally designated in the patent cited by the expression "conditioning zone," before reaching the evacuation station, where only a part of this current is removed, the rest returning to the source point, by the lower part of the bath, to be recycled there, thus constituting a second ringlike current, hereafter called the "homogenizing belt."
Said patent further indicates that, as observed in similar type flame furnaces, each of these two belts produces short circuits, i.e., some of the streams come back to the source point without having made the total travel described.
A drawback of furnaces of this type, whether flame or electric, resides in the risk of carrying materials directly from the composition batch to the evacuation station, without the various glassmaking processes being applied to them in a sufficiently complete manner.
Obviously, this risk is reduced by increasing the rate of recycling the material in each of the two major thermal belts but, on the other hand, the production energy cost is then increased. If one increases the homogenizing belt, for example, said recycling involves supplying, repeated at each cycle, of the amount of heat necessary to raise the temperature of the material from the temperature of the glass at the evacuation station, on the order of 1200.degree. C., to the fining temperature, on the order of 1500.degree. C. Now, the rate of recycling in this belt, equal to the ratio of the volumes of the return current and the draw, is usually at least on the order of 5, and often 10.
Besides the devices of this type, described above in a deliberately detailed way to show the phenomena observed in present tank furnaces and, at the same time, to define a vocabulary that is useful in the description of the present invention, there can be cited, as the prior art that can be taken into consideration in regard to the invention, devices comprising a partitioning of the tank, going even to assigning a particular compartment for each of the main operations to be used to provide glass of the desired quality.
Various patents, such as U.S. Pat. No. 1,820,248, FR Pat. No. 977 606, or GB Pat. No. 986 843, have thus proposed devices comprising a melting tank and a fining and conditioning tank, whose lower areas are connected to one another by a throat, and which are equipped with electrodes for heating by direct Joule effect through the molten glass. Although generally cooled, the throat can, however, also be heated electrically, optionally by an interconnection of the transformers feeding the two main compartments. These devices do not seem to have led to industrial developments meeting present needs in regard to fining, for reasons which, according to the teachings of the present invention, relate particularly to the insufficiency of confinement of the zone in which the fining is located and to convection phenomena.
Actually, it is known from these patents that the fining process extends both to the melting tank and to the conditioning tank.
An improvement in the control of the convection phenomena and confinement of the fining zone seems to be one of the aims of the U.S. Pat. No. 4,012,218, which proposes a device comprising a melting compartment, whose product is brought by overflow, by a surface current, to a fining compartment, where it receives an additional heating. The fined glass is separated by decanting, and is removed from the fining compartment at the bottom part. However, the quality of the fining is affected by the colder currents descending more rapidly along the walls than in the central region of the fining compartment. Further, obtaining of a homogeneous product that meets the standards contemplated by the invention is not described.
According to this same object of partitioning into compartments assigned to the main product operations, and especially to fining, the U.S. Pat. No. 2,733,111 provides, after a cell for melting vitrifiable materials, a fining cell, followed by a conditioning cell, the fining cell exhibiting the feature that the bath that it contains is kept constantly covered by an insulating foam layer whose upper skin is coagulated, thus forming a stable heat screen. This installation exhibits the drawback of not making it possible to attain fining and homogeneity levels corresponding to the requirements contemplated by the present invention. Actually, its fining cell, because of the arrangement of these various electrodes, some horizontal, some vertical, is a site of multiple thermal currents, which oppose one another and tend to make said cell function as a perfect mixer, leading to the delivery of a heterogeneous product in regard to length and conditions of treatment, i.e., the "thermal past," and, therefore, whose level of fining quality is limited. The semicircular shape of the conditioning cell does not lend itself well to creation of convection currents which, in the tank furnaces of the type described above, make it possible to deliver to the evacuation station a draw current consisting of glass protected from contact with the walls and bottom of refractory materials.